Hi LB tnx for ur thoughtful reply .My models are large! Danny and I shared our
work most of which was done over a year ago. I had two reasons sharing the
results. 1) You can accurately model a quad
much the same as a yagi. ie it performs the same in the air. 2) quantify the
distortion of a common feed
L. B. Cebik wrote:
> ON7NQ, Danny Mees, has done extensive modeling work on the 3-element
> 5-band Cubex quad, optimizing it for separate feed to avoid some of the
> problems associated with combined feed. His e-mail address is
> Danny did much of his modeling work, later confirmed on the antenna
> itself, on AO, and I did some confirmation modeling of his designs on
> NEC-4, with no great differences. He managed to increase gain and F-B--as
> well as getting a good match--on all bands. He even added a second
> director to 10 meters, since the constant spacing of the design sets the
> 10 meter elements quite far apart. I think his recommended modifications
> will eventually appear in the next Antenna compendium.
> I have done some extensive modeling of common and separate feed systems on
> 2-element quads, using a spider design rather than constant spacing.
> (QST ran a nice 2-element design with constant spacing back in the
> early 90s--exact reference not at hand as I write this.) The
> greatest common-feed interactions were 20-10 meters and 12-10 meters.
> When you model, you cannot claim to have covered all possible
> combinations; hence, I cannot say that there is no common-feed design that
> will achieve the same results as a comparable separate feed model.
> However, I found that it was much easier to achieve monoband results when
> using a separate feed system and the unused drivers were closed loops.
> Although it is relatively straight forward to get optimal performance from
> a 2-element quad in a multiband array, more than 2 elements requires
> constant spacing for a 2:1 frequency range--or the addition of high-band
> spreaders to make the spacing on those bands optimal. Like a multi-band
> Yagi, a multi-band quad is a compromise design in more than 2 elements.
> When designing quads via modeling in NEC, do not skimp on segments and
> keep them as aligned as possible. Using 7, 9, 11, 13, and 15 segments per
> side per band from 10 to 20 meters achieves this quite closely, but the
> resultant model is quite large. Auto-segmentation may or may not yield
> correct results--it is best to do one's own convergence testing.
> Of course, for those interested in modeling quads, Orr, Haviland, and
> Koszeghy are the standard references for basic designs and design
> LB, W4RNL
> L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
> 1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
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